204 Theory of Moving Electrons and Electric Charges. 



which all who adopt Maxwell's electrical scheme for free 

 aether must allow, expressions for the force (P 7 , Q', R/) which 

 acts on an element of volume of the free aether, and for the 

 force e (P, Q, R), ordinarily called electric force, which acts 

 on an electron e ; and it uses these forces for further develop- 

 ment of the theory. What Mr. Morton's computation 

 virtually does is to assume that the trail of each electron is 

 steady, and then to transfer to the electron itself the forcive 

 due to (P 7 , Q ; , W) acting on this aethereal trail. In the 

 special case of no radiation, and of velocities small compared 

 with that of radiation, this forcive can, as above explained, 

 be transmitted through the aether to the electron itself, and 

 be supposed there applied. But to so transmit it in general 

 is to miss the point of the theory, and, as Mr. Morton himself 

 remarks, to reach the absurdity that the force on a moving 

 charge depends not only on the state of the surrounding aether 

 but on the state of the aether at a distance. 



As regards the main subject of Mr. Morton's paper, it may 

 be of interest to state the following general theorem, Suppose 

 a system of charged conductors is in steady translatory motion 

 through the quiescent aether with velocity u, and let v repre- 

 sent the velocity of radiation in free aether : consider a cor- 

 relative system of conductors obtained by uniform geometrical 

 elongation of the actual system along the direction of motion 

 in the ratio of (1— u?lv 2 )~^ to unity, and find the electrostatic 

 distribution of the same charges on this system supposed at 

 rest : then the actual distribution of the charges on the 

 moving system will be exactly correlative, viz., equal charges 

 will exist on all corresponding elements of the two systems. 

 This proposition is, however, limited to the case in which 

 none of the bodies of the moving system are dielectrics, but 

 all are conductors. 



Finally, I take advantage of the present opportunity to 

 draw attention to some special points in which the analysis 

 of the second part of my memoir is incomplete. In § § 28, 29, 

 on the mechanical pressure of radiation, a statical forcive has 

 been overlooked ; when this is included the result practically 

 agrees with that given by Maxwell. In §§ 34-6, on the 

 material forcives in polarized media, the tractions on inter- 

 faces of transition remain to be developed, and the theory 

 may be otherwise improved. I hope presently to treat this 

 subject at length. 



Cambridge, June 4, 1896. 



